1. Field of the Invention
The present invention relates to a development device used for a copying machine or a laser beam printer employing an electrostatic recording system or an electrophotographic system for developing an electrostatic latent image, formed on an image bearing member, with developer including toner and carrier, and an image forming apparatus including the development device.
2. Description of the Related Art
Conventionally, an image forming apparatus such as a copying machine or a printer generally forms an electrostatic image onto an electrophotographic photosensitive member that is an image bearing member of a rotary drum, and develops the electrostatic image with developer to make the electrostatic image visible. So-called two-component developer containing non-magnetic toner and magnetic carrier has been widely used as the developer used in this type of the image forming apparatus.
A development system using two-component developer does not need to use magnetic toner. Therefore, this development system has advantages of providing stable image quality and durability of the apparatus, compared to the other development systems. However, deterioration in carrier is unavoidable in the two-component developer. Due to the repeated development, the non-magnetic toner is supplied in an amount used for the development, but the magnetic carrier is not used up and circulates in a developer container. Therefore, the deterioration of old carrier due to long-term durability has been unavoidable. In view of this, an image forming apparatus using the two-component developer has to discharge old carriers and supply fresh carriers for maintaining charging performance of the magnetic carrier.
Japanese Patent Laid-Open No. 2010-256701 describes a configuration in which developer containing toner in an amount used for a development operation of a development device and carriers in a fixed ratio is supplied to a development device, and surplus developer in the development device is discharged to a waste developer storage container arranged at the outside of the development device. Specifically, the developer is supplied, and almost at the same time, the surplus developer is discharged. With this, fresh carriers are supplied with the supply of toner, whereby the performance of the whole developer can be stabilized.
The development device that supplies and discharges developer includes a development chamber that supplies the developer to a photosensitive drum, and a stirring chamber that supplies the developer to the development chamber, wherein the development chamber and the stirring chamber are separated by a partition wall. A conveyance screw is provided in the respective chambers. The development chamber and the stirring chamber communicate with each other at both ends in the longitudinal direction, and the developer that is stirred and conveyed by the conveyance screw is circulated. Two-component developer is supplied from the upstream side in the conveyance direction by the conveyance screw in the stirring chamber, and the developer circulated in the developer container is discharged, little by little, from a discharge opening formed on an end surface at the downstream side.
As illustrated in FIG. 5, a conveyance screw 6 includes a helical conveyance portion 11 that conveys the developer in the direction of circulating the developer to the discharge opening, and a helical return screw 10 that is coupled to the downstream portion of the conveyance portion 11 for conveying the developer in a direction opposite to the conveyance direction of the conveyance portion 11. The return screw 10 pushes back most of the developer conveyed by the conveyance portion 11 toward the discharge opening in order to prevent excessive discharge of the developer from the discharge opening. The developer climbing over the return screw 10 is conveyed by a discharge screw 9 and discharged from the discharge opening.
In the configuration including the return screw 10 as described above, a small clearance (about 1.0 mm) is formed between the return screw 10 and an inner wall of the developer container 2 as illustrated in FIG. 13. The clearance is formed to prevent interference between the return screw 10 and the inner wall of the developer container 2, considering the dimension tolerance of the return screw 10 and the developer container 2. When the return screw 10 interferes with the inner wall of the developer container 2, an aggregate of the developer might be generated on the interference portion, and the generated aggregate might cause an image failure such as stain by toner.
On the other hand, when the clearance is formed between the return screw and the developer container as described above, the developer might leak from the clearance illustrated in FIG. 13. When the leakage of the developer occurs, the developer is discharged upon driving the development device, with the result that the developer in the development device might be exhausted. As a result, the developer coating on the development sleeve becomes insufficient, so that an image density might be uneven.
The phenomenon of the leakage of the developer is likely to occur at an upper part of the clearance as indicated by C portion in FIG. 13. On the other hand, the lower part of the clearance is packed by the developer due to its own weight, whereby the leakage of the developer at this portion hardly occurs.
In view of this, an apparatus including a non-contact restriction member provided for the return screw 10 at the clearance has been proposed (Japanese Patent Laid-Open No. 2010-237328). However, it is difficult to completely prevent the leakage of the developer, since a clearance is also formed between the restriction member and the container.